The present invention relates to a mechanical shaft joint boot made of a resin comprising a bellows having four to seven crests between a large-diameter ring and a small-diameter ring (hereinafter called "boot"), and particularly relates to a boot suitable for a shaft joint of a constant velocity type for an automobile.
In this specification, the crests are numbered first, second, . . . in order from the large-diameter ring of the bellows, the troughs are numbered in the same order as the crests, and the radius of curvature (R.sub.i) shows an external curvature of the top of a crest i.
In general, a boot made of a resin having a bellows of the above-described type has conventionally been so formed that the radius, of curvature of all the crests are identical, as shown in FIG. 1 (a five-crest type). Such a boot has conventionally been formed by blow molding and thus the thicknesses of the material at the crests and at the troughs have not been uniform, the material at the crests (particularly their tops) being thinner than at the troughs (refer to the black circles shown in FIG. 5).
It is estimated from the results of various investigations conducted by the inventors that this is due to the duplication of one or more of the causes described below.
(a) Since the thickness is rapidly reduced from the trough to the top of the crest, the distortion of elongation and compression is concentrated at the top.
(b) The elongation distortion is particularly concentrated at the crest which lies at the center of the bellows or at crests lying between the large-diameter ring and the center (the second and third crests in the boot shown in FIG. 1). (the cause is not known, refer to "the X-ray fluoroscopic view showing the state of a boot during a crossing movement" in FIG. 2).
(c) A bending (kinking) phenomenon is produced at the central crest of the bellows or at a crest lying between the large-diameter ring and the center (FIG. 3). It can be conjectured that the production of this kinking phenomenon is caused by the rapid change in compression and elongation which is attributed to the poor movement of the greatly enlarged crest toward the compressed side.